An air preheater (APH) is a entire term to explain any piece of equipment designed to heat air before another process (for example, combustion in a boiler) with the most important objective of increasing the thermal efficiency of the path. They may be used alone or to replace a recovery heat system or to replace a steam coil.
In particular, this article describes the combustion air preheaters used in large boilers found in thermal power stations producing electric power from e.g. fossil fuels, biomasses or waste.
The purpose of the air preheater is to get better the heat from the boiler flue gas, which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack.
Air pre-heater transfer heat from flue gas to air by means of a rotary matrix in which heat is absorbed by the heating elements passing through the hot gas stream and transferred to the combustion air stream.